The present invention relates to a blockable bone plate with secured bone screws which serve in particular as a reconstruction system for the maxillofacial region of the human skull, for example for the lower jaw. For the time being, the shorter terms xe2x80x9cplatexe2x80x9d, xe2x80x9cscrewxe2x80x9d and xe2x80x9cplate-screw connectionxe2x80x9d are used. Such plates are used, for example, to strengthen weakened and damaged bone structures. In the case of large continuity defects, the plates have to provide stability over long periods of time and take up the loads which the missing bone would have supported. Plates of this type consist in their simplest form of a straight elongate branch with a multiplicity of plate members, in each of which a screw hole is provided for receiving a screw. The plate can be tailored to the required length, which is done by cutting off surplus plate members, and can be bent to adapt it to the local anatomical situation.
The bone plate and a plate-screw connection have to satisfy the following requirements:
high degree of strength and stability of the bone plate;
high degree of rigidity of the plate-screw connection;
flexibility of the plate in all planes with minimal loss of strength both in the plate plane and also around the plate plane, including as torsion;
possibility of applying the various biological tissue types on the plate;
least possible surface pressure of the plate on the periosteum; and
securing (so-called blocking) of the screws passing through the plate in order to prevent the screws from coming loose.
In order to keep the surface pressure of the plate on the periosteum to a minimum, U.S. Pat. No. 5,810,823 proposes arranging spacer elements with an internal thread which are fixed on or can be fitted onto the lower surface of the plate and which sit on the bone and are passed through by the threaded shanks of the screws. Spacer elements whose position can be varied also permit an orientation of the screws which deviates from the vertical. Although this construction avoids the plate pressing on the periosteum over a large surface area, increased pressure nevertheless arises at the points of contact of the spacer elements. The relatively small spacer elements that can be fitted complicate the surgical procedure. Regardless of whether the spacer elements are fixed or can be fitted in place, they do not provide any improved securing of the screws against their inadvertently coming loose. WO 96/39975 describes a blocked plate-screw connection in which recessed securing elements are provided on the lower surface of the plate, which can be fitted in the area of the screw holes. The threadless neck portion of the screw shank passes through the individual securing element, said screw shank having a smaller diameter than the threaded part. The threaded part can firstly be passed through the securing element and screwed into the bone, the screw head coming to lie in a countersink on the upper surface of the plate. After heat treatment, the securing element shrinks so that the plate is held at a distance from the periosteum and the screw is secured against unscrewing. This system requires the special securing elements and devices and is therefore expensive to manufacture and handle.
The company brochure xe2x80x9cSYNTHES(copyright)xe2x80x94THORP Reconstruction Setxe2x80x9d from STRATEC Medical, Waldenburg, Switzerland, discloses a plate system in which anchoring screws are first screwed through the screw holes of the plate into the bone. An expansion screw is screwed into the slotted head of the anchoring screw which sits in the screw hole. In this way, the head of the anchoring screw presses against the wall of the screw hole in the plate. The anchoring screw is thus secured against unscrewing and the plate is held as it were at a distance from the periosteum. To ensure that the screw holes in the plate do not narrow upon bending, bend inserts are provided as insertable cores. The relatively large number of parts means that the operation takes longer and entails more instrumentation and is as a whole made more difficult.
A further development of a blockable plate-screw connection is known from U.S. Pat. No. 5,709,686. The bone plate has a plurality of holes provided in the direction of the longitudinal axis of the plate and intended for the passage of screws. The screw hole is of oval shape, its main axis lying in the longitudinal axis of the plate. On the upper surface of the plate, the screw hole is surrounded by a spherical countersink. Over the shorter minor axis of the screw hole there is a partial internal thread whose turns each run out to the main axis of the oval screw hole. The internal thread serves to receive a threaded portion which lies under the head of the screw and which in diameter is widened compared to the threaded shank of the screw. The screw hole opens toward the lower surface of the plate in the area of both ends of the main axis, i.e. outside the internal thread on the longitudinal axis of the plate.
In the clinical application of the plate, the threaded shank of the screw protrudes into the bone while the threaded portion with the internal thread engages in the screw hole. The plate is thus supported and is not pressed by the tensile force of the screw onto the periosteum. The screw is secured against inadvertent unscrewing by the threaded connection between the internal thread in the screw hole and the threaded portion of the screw. Plate and screw are blocked. In the event of eccentric application of a screw without threaded portion, the countersink around the screw hole permits a compression between bone compartments. Screws without a threaded portion for blocking can also be screwed in in a direction deviating from the vertical. Greater clearance for this exists in the direction of the longitudinal axis of the plate as a result of the oval hole shape and the only partial internal thread in the screw hole.
The plate-screw connection according to U.S. Pat. No. 5,709,686 permits satisfactory blocking and an additional securing of the screws, but there are still serious disadvantages, for example:
To prevent deformations of the internal thread in the screw holes when bending the plate, bend inserts again have to be used.
Even at low torques when the screw is being screwed in, the internal thread in the screw hole can be overscrewed. With standard screwdrivers, such critical torques can be quite easily applied. A damaged plate is unusable; a new plate has to be used and if appropriate also a new screw.
In a blocked plate-screw connection, the inserted screws can only be positioned vertically, which makes it necessary to use exactly vertical drill guides.
Because of the overall geometry of the plate holes, machining has to be done from two directions, which makes the plate production more expensive.
In view of the stated inadequacies of the blockable bone plates known to date, with secured screws, the object of the invention is to propose an improved blockable plate-screw connection. The aim here in particular is to manage without third parts such as inserts or expansion screws for blocking and bend inserts. The connections are intended to be less sensitive to overscrewing and angled offset of the screws in relation to the vertical and the longitudinal axis of the plate. It is desirable, within the plate-screw connection, to obtain a high release torque for the screws in order thereby to more effectively prevent the screws from coming loose. The clinical application is to be made as straightforward as possible for the operator. It must still be possible to perform osteosynthetic bone compression and to use the connection with conventional screws unblocked. Finally, the parts of the connection must be able to be manufactured economically in serial production.
The further development of the blockable bone plate according to the invention is based on a bone plate and screws with a blocking thread according to the generic type from U.S. Pat. No. 5,709,686. The plate consists of a plurality of plate members which are connected to each other via webs. A screw hole is provided in at least some plate members, preferably in each plate member, and is surrounded on the upper surface of the plate by a spherical countersink. Provided internally in the screw hole there is an engagement contour which consists of contour valleys and adjacent remaining contour peaks partially running in a horizontal and radial peripheral direction on the wall of the screw hole. The diameter of the countersink for receiving the screw head is greater than the clear width of the engagement contour. This runs out to the edges of the engagement contour so that smooth uncontoured wall areas are left there in the screw hole. In relation to the threaded shank of the screw, the screw hole is shaped as an oblong hole. The engagement contour is preferably produced by milling and has for example a pointed, round, trapezoidal or serrated configuration.
The screw intended for blocking has, under the screw head, a blocking thread which has the same diameter as or a greater diameter than the thread on the threaded shank which is intended for engagement in the bone. When fixing the plate on the bone, the screw passes through the screw hole with its threaded shank, and the thread of the threaded shank screws into the bone. In the final phase of screwing-in of the screw, the blocking thread under the screw head engages in the engagement contour in the screw hole. As the blocking thread with its helical course and its pitch is not complementary in form to the engagement contour, deformation occurs on both, resulting in a connection resistant to loosening. A blocking of reduced strength can be obtained if on the screw there is a complete thread whose uppermost turns run jammed underneath the engagement contour.
Particular advantages in the osteosynthesis of fractures on curved bones, for example a lower jaw, are obtained using the blocking according to the invention on a bone plate with a plate longitudinal axis in the shape of an arc of a circle. The bone plate consists at least of a main segment in the shape of an arc of a circle. For special purposes, the main segment is provided with a lateral segment which is attached to it at one end or at both ends and lies in the plate plane, and which lateral segment is straight or curved. Some screw holes on the bone plate can be designed not for blocking, but as cylindrical standard screw holes or as oriented compression holes. The bone plate in the shape of an arc of a circle preferably acquires its shape without forming, so that the plates reach the surgeon without initial weakening caused by bending stresses.
By virtue of the invention, a blockable plate-screw connection is now available with the following advantages:
The plate-screw connection is efficient in clinical application since no third parts are needed for blocking. Also, no bend inserts are needed when bending the plate. The plate tends to bend in the area of the webs connecting the individual plate members. However, even in the event of bending within the screw hole, the function of the blocking is not impaired.
During blocking, no forces act on the periosteum, so that the latter is protected. The plate can be positioned at a distance from the bone similar to a fixator.
The internal engagement contour in the screw hole also permits a slight angular offset of the inserted screw in relation to the vertical and to the longitudinal axis of the plate. This is achieved without any functional loss of the blocking.
Upon generation of compression, the screws are not subjected to tensile stress since the blocking of the screw in the plate prevents any axial movement between plate and bone.
The inserted screws are better protected against inadvertent loosening because a high release torque has to be applied. This is due to the deformations during blocking on the internal engagement contour on the screw hole and on the associated thread of the screw.
The risk of overscrewing and thus of damage to the plate is reduced. In conventional plates, the screws are overscrewed even at low insertion torques which can be applied effortlessly with standard screwdrivers.
The engagement contour arranged internally in the screw holes causes substantially less notch effect than a conventional internal thread, as a result of which there is less risk of the plate breaking.
The plate can be fitted with different screws, namely those with which blocking is obtained and those without blocking. In the latter case, the screw holes function as neutral compression holes for receiving the screw head. This, for example, for applying small screws in order to fasten a bone compartment. Here, the screw can also be inserted at an inclination.
Since the complete geometry of the screw holes in the plate can be worked from one side, retooling is not required during machining, thus resulting in overall cost-effective production which can also be executed with precision at a reasonable cost.
In the configuration of the bone plate with a longitudinal axis of the plate in the shape of an arc of a circle (at least in the main segment if the bone plate is provided with a lateral segment attached at one end or both ends), after bending the webs out of the plate plane, as it were over the surface, it is possible to adapt the bone plate closely to the bone. The additional bending in the plate plane is dispensed with, that is to say the setting or twisting over the edge. By means of the circular arc-shaped starting geometry, the bone plate assumes the inclination of a circular section of the jacket surface of a cone. This reduces the surgical effort when adapting the plate and also reduces the losses of strength thereof because of minimal deformation. There is ideal adaptation of the bone plate in its longitudinal extent and the inclination over the plate width.